Nanoparticles and systems and methods for synthesizing nanoparticles through thermal shock

ABSTRACT

Systems and methods of synthesizing nanoparticles on substrates using rapid, high temperature thermal shock. A method involves depositing micro-sized particles or salt precursors on a substrate, and applying a rapid, high temperature thermal pulse or shock to the micro-sized particles or the salt precursors and the substrate to cause the micro-sized particles or the salt precursors to become nanoparticles on the substrate. A system may include a rotatable member that receives a roll of a substrate sheet having micro-sized particles or salt precursors; a motor that rotates the rotatable member so as to unroll consecutive portions of the substrate sheet from the roll; and a thermal energy source that applies a short, high temperature thermal shock to consecutive portions of the substrate sheet that are unrolled from the roll by rotating the first rotatable member. Some systems and methods produce nanoparticles on existing substrate. The nanoparticles may be metallic, ceramic, inorganic, semiconductor, or compound nanoparticles. The substrate may be a carbon-based substrate, a conducting substrate, or a non-conducting substrate. The high temperature thermal shock process may be enabled by electrical Joule heating, microwave heating, thermal radiative heating, plasma heating, or laser heating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalApplication No. 62/523,646, filed on Jun. 22, 2017, the entire contentsof which are incorporated herein by reference.

BACKGROUND Technical Field

This disclosure relates to nanoparticles synthesized or constructed on asubstrate of interest through thermal shock, their methods ofconstruction, and corresponding systems.

Related Art

The synthesis of nanoparticles of metal and metal compounds hasattracted a massive amount of attention because of their good catalyticactivity. The conventional synthesis method—wet chemistry—involvescomplex chemical reactions where precise control of reaction conditionsis required. This makes it challenging to synthesize uniform small-sizenanoparticles. Moreover, uniformly dispersing nanoparticles on asubstrate is even more challenging. Slight differences in reactionconditions can drastically change the morphology of the end product,which may be detrimental to the performance of the catalyst. Thisproblem may become more severe and relevant when the catalyst hasdimensions on the nanoscale level.

SUMMARY

Existing challenges associated with the foregoing, as well as otherchallenges, are overcome by methods for synthesizing nanoparticles on asubstrate, and also by systems and apparatuses that operate inaccordance with these methods.

In aspects, this disclosure features a method of forming nanoparticleson a substrate. The method includes depositing micro-sized particles orsalt precursors on a substrate, and applying a rapid, high temperaturethermal shock to the substrate and the micro-sized particles or the saltprecursors to cause the micro-sized particles or the salt precursors toself-assemble into nanoparticles on the substrate.

In another aspect, this disclosure features a system for synthesizingnanoparticles on a substrate. The system includes a rotatable memberthat receives a roll of a substrate sheet on which is depositedmicro-sized particles or salt precursors, and a motor that rotates therotatable member so as to unroll consecutive portions of the substratesheet from the roll. The system also includes a thermal energy source,such as a thermal radiation source or a direct Joule heating source,that repeatedly applies a short, high temperature radiation pulse toconsecutive portions of the substrate sheet that are unrolled from theroll by causing the motor to rotate the first rotatable member to causethe micro-sized particles or the salt precursors to self-assemble intonanoparticles on consecutive portions of the substrate sheet.

In yet another aspect, this disclosure features a composite, such as afilm. The composite includes a substrate and a plurality ofnanoparticles formed on the substrate from a micro-sized particle orsalt precursors exposed to a rapid, high temperature thermal shock.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the disclosure andtogether with a general description of the disclosure given above, andthe detailed description of the embodiment(s) given below, serve toexplain the principles of the present disclosure.

FIG. 1A is a schematic diagram illustrating an example method forconstructing nanoparticles on a substrate in accordance with thisdisclosure;

FIG. 1B is a schematic diagram illustrating an example mechanism fornanoparticle formation in accordance with this disclosure;

FIGS. 1C-1H are example scanning electron microscope (SEM) imagesillustrating nanoparticles of different materials formed in RGO networksin accordance with this disclosure;

FIGS. 2A and 2B are schematic diagrams illustrating metallicnanoparticles, which are synthesized from salt precursors, uniformlydeposited on carbon nanofibers in accordance with this disclosure;

FIGS. 2C-2G are electron microscopy images illustrating different typesof metallic nanoparticles uniformly synthesized on carbon nanofibers inaccordance with this disclosure;

FIG. 3 shows SEM images and STEM elemental maps illustrating uniformlyseparated multi-metallic nanoparticles in accordance with thisdisclosure;

FIGS. 4A-4F show schematics and morphology of the synthesis forexamplary metal compound nanoparticles on RGO sheets in accordance withthis disclosure;

FIGS. 5A-5C are schematic diagrams illustrating systems for the scalablenanomanufacturing of nanoparticles through roll-to-roll manufacturing inaccordance with this disclosure;

FIGS. 6A and 6B are graphs illustrating the properties of synthesizedCoS nanoparticles;

FIG. 6C is a schematic diagram illustrating water splitting apparatushaving electrodes made of synthesized CoS nanoparticles;

FIGS. 6D and 6E are graphs illustrating the water splitting performanceof the synthesized CoS nanoparticles;

FIGS. 7A and 7B are graphs illustrating the electrochemical performanceof the synthesized Si nanoparticles as the anode of Li-ion battery inaccordance with this disclosure;

FIG. 7C is a series of images illustrating 3D printing of GO and GO-Siribbons and the various manipulations of those ribbons in accordancewith this disclosure;

FIG. 8 is a flow diagram illustrating a method of synthesizing ametallic nanoparticle on carbon nanofibers from salt precursors inaccordance with this disclosure;

FIG. 9 is a flow diagram illustrating a method of synthesizing ametallic nanoparticle on a substrate from a micro-sized metallicparticle in according with this disclosure; and

FIG. 10 is a schematic diagram illustrating particle size-control bymodifying the surface of the substrate to tune the wetting behavior ofthe substrate with the nanoparticles in accordance with this disclosure.

DETAILED DESCRIPTION

Embodiments of the systems and methods of synthesizing nanoparticles onsubstrates are described in detail with reference to the drawings, inwhich like or corresponding reference numerals designate identical orcorresponding elements in each of the several views.

This disclosure relates to low cost, simple, ultra-fast synthesis ofnanoparticles for nanocatalysis, which is beneficial for the developmentof high performance nanocatalysts used for energy conversion andelectrochemical processes, such as water splitting, fuel cells, metalair batteries, and other catalytic reactions, such as biomassconversion, ammonia oxidation, and so on.

FIG. 1A illustrates an example method for constructing nanoparticles(e.g., metallic or semiconductor nanoparticles) on a substrate (e.g.,reduced graphene oxide (RGO)) from micro-sized particles. Themicro-sized particles in the substrate are self-assembled intonanoparticles when heated by direct Joule heating. The micro-sizedparticles may include or may be made of bulk materials that are largeaggregates or raw minerals. The micro-sized particles may include or maybe made of various elements, including, for example, aluminum, tin,gold, palladium, iron, nickel, silicon, or any combination of thesemicro-sized particles. The nanoparticles may include or may be metallic,ceramic, inorganic, semiconductor, compound nanoparticles, or anycombination of these nanoparticles. In some embodiments, the synthesizednanoparticles may have an average diameter of between about 1.0 nm andabout 30 nm. In other embodiments, the synthesized nanoparticles mayhave an average diameter outside this range, e.g., between about 30 nmand about 100 nm.

The substrate may be a carbon-based substrate, a conducting substrate,or a non-conducting substrate. The carbon-based substrate may be areduced graphene oxide substrate or a carbon nanofiber substrate. Insome embodiments, the surface of the substrate may be treated to modifythe wetting behavior and thereby change the nanoparticle size. Forexample, the surface of the substrate may be coated with an oxidecoating by an atomic layer deposition (ALD) process or a solutionprocess.

FIG. 1B illustrates an example mechanism for nanoparticle formation inaccordance with embodiments. Agglomeration and coalescence of thenanoparticles, which is driven by surface energy minimization, issuppressed by the defects 130 of RGO in-plane and RGO layersout-of-plane. The defects 130 serve as barriers for atom migration tokeep the uniformly distributed nanoparticles 120 separated. Duringheating or melting 125, the particles within the matrix are protectedfrom oxidation by the RGO sheets, due to the impermeability of RGO to O₂and H₂O.

The RGO sheets work well as a host material because of their defectsites and high melting temperature. For example, the RGO sheets may bestable up to 3300 K. Thus, the micro-sized particles melt 125 uponheating and self-assemble into nanoparticles 120 due to the confinementby the defects 130 of the RGO sheet. FIGS. 1C-1H show images of examplemetallic nanoparticles distributed on RGO substrates. In embodiments,the thermal shock may be provided by other suitable heating methodsincluding (a) radiation heating, (b) microwave heating, (c) plasmaheating, and (d) laser heating. In embodiments the temperature of thethermal shock may range between 500K and 3000K. In other embodiments,the temperature of the thermal shock may be outside that range.

In embodiments, the substrate may be formed and the micro-sizedparticles may be deposited on or applied to the substrate to form a filmaccording to any suitable procedures. For example, as illustrated by themethod of FIG. 9, an Al-GO film may be formed by preparing a GO ink(block 902), preparing an Al-GO solution (block 904), preparing an Al-GOfilm (block 906), and reducing the Al-GO film (block 908).

In some embodiments, the Hummer's method may be employed for thepreparation of GO ink (block 902). First, a suspended solution of amixture of natural graphite flakes (e.g., 1.5 g) and KMnO₄ (e.g., 9 g)in acid of H₂SO₄/H₃PO₄ (e.g., 200 mL with a volume ratio 9:1) isprepared. For better dispersion, the solution may be heated (e.g.,heated to 50° C.) while stirred continuously for an appropriate period(e.g., 12 hours). After achieving a uniform composition, the solutionmay be cooled down to room temperature before being poured onto ice(e.g., 200 mL) mixed with H₂O₂ (e.g., 3 mL). Subsequently, an HClsolution (e.g., a 100 mL 30% HCl solution) with a DI water bath may beused to wash away unwanted flakes. The resulting GO solution may have aconcentration of 2.5 mg mL⁻¹ after diluting it in distilled water.

Next, the Al-GO solution is prepared (block 904). The Al powders ink maybe prepared by adding micro Al powders (e.g., 30 mg of 99.5%,Sigma-Aldrich) into ethanol (e.g., 12 mL), followed by sonication (e.g.,1 minute). The concentration of the prepared Al powders solution may be2.5 mg/mL. The Al-GO solution may be prepared by mixing the as-preparedGO solution and Al powders solution with a weight of, for example, 1:1,followed by shaking (e.g., 1 minute) on a tube vertex mixer and thensonication (e.g., 10 seconds). Thus, a high quality Al-GO solution withGO sheets and monodispersed Al powders can be obtained since the GOsheets serve as a surfactant to disperse Al micro powders to form auniform Al-GO suspension.

Next, the Al-GO film is prepared (block 906), e.g., via vacuumfiltration. A freestanding Al-GO film (e.g., 35 mm in diameter) may beobtained by filtering the Al-GO solution (e.g., 6 mL) through a membrane(e.g., 0.65 μm pore-sized membrane). The film on the membrane is thendried in the air. The Al-GO film may then be detached from the membranenaturally. Then, the Al-GO film is reduced (block 905). Thermalpre-reduction of the Al-GO film may be carried out in a furnace. Forexample, thermal pre-reduction of the Al-GO film may be carried out in atube furnace under H₂/Ar (5%/95%) atmosphere, held at 300° C. for 10 minwith a ramping rate of 5° C./min.

An appropriate structure, such as glass slides, may be prepared to holdthe Al-RGO film. Two copper ribbons may be attached at each side of theglass mount, functioning as the connections to an external circuit andheat sinks. Silver paste may be applied to both ends of the Al-RGO filmas electrodes, forming an Ohmic contact to the later annealed film.Then, a rapid, high temperature thermal shock is applied (block 910) tothe Al-RGO film by applying a voltage across the two copper ribbons tosynthesize Al nanoparticles on the RGO film.

In other embodiments, nanoparticles may be synthesized on substratesfrom salt precursors. The salt precursors may include or be made ofmetal chloride, metal nitrate, metal acetate, or any combination ofthese salt precursors. FIG. 2A is a schematic diagram illustrating amethod of synthesizing metallic nanoparticles 206, e.g., Pdnanoparticles, on carbon networks 202 or carbon nanofibers (CNFs) fromsalt precursors 204, e.g., PdCl₂. The method involves uniformlydepositing or forming salt precursors (e.g., MCl_(x)H_(y) or PdCl₂) ontoa conducting matrix (e.g., the carbon nanofibers 202). For example, thesalt precursor PdCl₂ may be formed on a CNF surface by a dip-coatingmethod. The precursor-loaded is then treated with a rapid thermal shockby Joule heating leading to decomposition of the precursors andnucleation of uniform nanoparticles 206, which are well-dispersed on theCNFs.

FIG. 2B illustrates a mechanism for controlling nanoparticle size bycontrolling the thermal shock durations. The longer high temperaturethermal shock, e.g., 1 s, leads to particle agglomeration andcoalescence at high temperature, thus increasing the particle sizes. Afaster shock, e.g., 5 ms, creates smaller particle sizes. In otherembodiments, the duration of the thermal shock may be greater than 1 sor less than 5 ms to control the nanoparticle size. For example, theduration of the thermal shock may be 5 s, 2 s, 100 ms, or 1 ms.

FIGS. 2C-2G are example electron microscopy images of metallicnanoparticles uniformly synthesized on CNFs by thermally shocking saltprecursor-loaded CNFs. FIGS. 2C and 2D are scanning electron microscopy(SEM) and transmission electron microscopy (TEM) images, respectively,of Pd decorated CNFs after 1 s and 5 ms thermal shocks, respectively, atabout 2000 K. A large number of Pd nanoparticles are formed anduniformly distributed along the CNFs. The average size of Pdnanoparticles formed by the 5 ms shock (e.g., about 4 nm) is muchsmaller compared to the 1 s shock (e.g., about 27 nm), demonstrating theeffect of shock time on the synthesis of ultrafine nanoparticles. FIGS.2E-2G are electron microscopy images of synthesized nanoparticles onCNFs for Au, Fe, and Sn, respectively, which demonstrates that the hightemperature thermal shock method of this disclosure may be applied to awide range of elements.

In embodiments, salt precursors may be deposited on or in a CNF film toform a precursor-loaded film to be heated according to any suitableheating method. For example, nanoparticles dispersed on a film may beformed by preparing a film, depositing a salt precursor on the film toobtain a salt precursor-loaded film, and then applying a thermal shockto the salt precursor-loaded film.

In embodiments, a CNF film may be prepared according to the examplemethod of FIG. 8. The nanofibers are prepared by electrospinning (block802) a polymer solution (e.g., polyacrylonitrile (PAN) with aconcentration of 8 wt % in dimethylformamide) at a designed spinningcondition (e.g., a voltage of 10 kV, a spinning distance of 15 cm, and arate of 1 mL/hour). The resulting nanofiber mat is then stabilized(e.g., by placing the nanofiber mat in air at, for example, 260° C. for5 hours) and carbonized (e.g., by placing the stabilized nanofiber matin an argon atmosphere at, for example, 600° C. for 2 hours) to obtainthe CNFs (block 804).

Next, a salt (e.g., PdCl₂) is dissolved into a solution (e.g., water orethanol) (block 806) and the resulting salt solution is deposited orapplied into or onto the CNFs (e.g., by dipping, soaking, or vacuumfiltration) (block 808) and dried (block 809). The resulting film (e.g.,PdCl₂-CNF film) is then exposed to rapid, high temperature thermal shockto synthesize nanoparticles (e.g., Pd nanoparticles) on the CNFs.

FIG. 3 shows TEM images and scanning transmission electron microscopy(STEM) elemental maps of unary (Pt, Pd, Au, and Fe), binary (PtNi, PdNi,AuCu, and FeNi), ternary (PtPdNi, AuCuSn, and FeCoNi), senary(PdSnCoFeNiCu), and octonary (PdNiCoAuCuFeSnPt) nanoalloys ornanoparticles, which may be synthesized by adding multiple salts toprecursor solutions and performing the thermal shock process. As shownin FIG. 3, the nanoparticles are evenly dispersed across the carbonsupport. The nanoparticles may also possess size uniformity withdiameters greater than 5 nm, regardless of the elemental compositions.Moreover, the nanoparticles are of nanoscale dimensions in fcc crystalstructures. While FIG. 3 shows STEM elemental maps of specific uniformlyseparated multi-metallic nanoparticles (MMNPs) on carbon nanofibersubstrates, other multi-metallic nanoparticles with different metalelements and combinations of metal elements may be synthesized.

FIG. 4A illustrates an example process of synthesizing inorganiccompound nanoparticles on reduced graphene oxide. In some embodiments,the inorganic compound nanoparticles may be CoB, CoS, or FeS₂.Synthesized FeS₂ nanoparticles derived from iron pyrite are highlyefficient and stable electrocatalysts for water splitting. FeS₂ powders404, which may be derived from iron pyrite, and graphene oxide flakes402, which may be exfoliated from graphite through an improved Hummers'method, may be used as raw materials to fabricate a micro-FeS₂-RGO film.The nano-FeS₂-RGO film is in situ synthesized by directly joule heating412 the as-prepared micro-FeS₂-RGO film to a high temperature (e.g.,approximately 2470 K) in a short time (e.g., approximately 12 ms). Afterthermal shock treatment, FeS₂ nanoparticles 410 (e.g., having a size of10-20 nm) are uniformly distributed on RGO nanosheets, as illustrated inFIG. 4A.

In the rapid heating process 412, in which the micro-FeS₂-RGO film maybe heated up to 2470 K, FeS₂ powders decompose into Fe atoms 406 and Satoms 408. The Fe atoms 406 and S atoms 408 then diffuse 414 within theRGO matrix but remain in between the RGO layers under high temperature,thereby benefiting from the impermeability of RGO and the encapsulationeffect of the RGO film. As rapid cooling 416 takes place, the Fe atoms406 and S atoms 408 renucleate around the defects on the basal plane ofthe RGO nanosheets and crystallize into ultrafine FeS₂ nanoparticles410. The FeS₂-RGO 3D nanostructure helps to maintain good mechanicalintegration and rapid electron transport of FeS₂ nanoparticles embeddedin the RGO nanosheets.

The methods of this disclosure enable in situ synthesis of FeS₂ andother inorganic compound nanoparticles in an ultrafast, cost-effective,and scalable approach. FeS₂ nanoparticles transformed from iron pyritethrough ultrafast thermal shock can be used as catalysts to split water.Benefiting from the ultrafine FeS₂ nanoparticles and the robust FeS₂-RGO3D structure, the as-synthesized nano-FeS₂-RGO exhibits remarkableelectrocatalytic performance for HER with, for example, only 139 mVoverpotential to achieve 10 mA cm⁻² current in 0.5 m H₂SO₄ solution forlong-term operation. This strategy may also be applicable to synthesizeother transition metal dichalcogenides and may be extended to ternary ormulticomponent compounds.

FIG. 4B show example transmission electron microscopy (TEM) images ofthe structure of the as-synthesized FeS₂ and RGO composite. TEM image(a) shows the typical spherical morphology of FeS₂ nanoparticles with anaverage size of 10-20 nm distributed on wrinkled 2D RGO nanosheets.Enlarged TEM image (b) illustrates that FeS₂ nanoparticles are uniformlyembedded in the RGO nanosheets. High-resolution TEM image (c) showslattice plane spacing of 0.22 nm, which corresponds to the crystalplanes of the pyrite structure, indicating excellent crystallinity ofthe as-synthesized FeS₂ nanoparticles. The high-angle annular dark-fieldTEM image (d) of an FeS₂ nanoparticle and the corresponding EDXelemental mapping images (e) and (f) of as-synthesized FeS₂nanoparticles show the distribution of Fe and S elements over the wholenanoparticle on the RGO nanosheet.

FIG. 4C show example transmission electron microscopy (TEM) images ofsynthesized CoS nanoparticles in accordance with this disclosure. TEMimage (a) of FIG. 4C shows the uniform distribution of CoS nanoparticleson RGO nanosheets. Enlarged TEM image (b) shows CoS nanoparticles withan average size of 20 nm embedded in two-dimensional (2D) RGO nanosheetswith strong adsorption. As shown in high resolution TEM image (c), theCoS nanoparticle may be coated with graphene layers, e.g., with athickness of approximately 2 nm, corresponding to approximately sixlayers of graphene, which may be formed by the recrystallization ofactive carbon atoms on RGO nanosheets encapsulated in the whole RGO filmduring the ultrafast high temperature treatment. Carbon coatings oncatalysts may play a role in the catalytic performance of catalysts insolution. A coated carbon shell on a catalyst with an appropriatethickness can prevent the catalyst from being directly exposed insolution and avoid corrosion during long term operation, while stillensuring efficient electron transport and maintenance of effectivecatalytic activity.

FIG. 4C shows an example atomic resolution TEM image (d) of acobalt-sulfide nanoparticle, which shows crystalline planes with aninterplanar distance of 0.19 nm. This can be ascribed to planes of CoS,confirming the excellent crystallinity of the synthesized CoSnanoparticles. The example high-angle annular dark-field TEM image (e)and the corresponding EDX elemental mappings (f) and (g) of CoSnanoparticles illustrate the homogeneous distribution of Co and Selements over the whole nanoparticle on the RGO nanosheet, demonstratingthe uniform atomic mixture of Co and S atoms in as-synthesizedparticles.

FIG. 4C shows example cobalt acetate and thiourea, forming approximately1 μm clusters on graphene oxide film. The large-scale graphene oxidefilm containing cobalt acetate and thiourea mixture may be prepared by asimple casting method, as illustrated in the inset image of FIG. 4D. Themorphology of as-prepared cobalt-sulfide nanoparticles is shown in FIG.4E, which reveals that the cobalt-sulfide nanoparticles areapproximately 20 nm in diameter and uniformly distributed on RGOnanosheets. A close view image in the inset of FIG. 4E shows that mostof the nanoparticles are either spherical or ellipsoidal.

FIG. 5A is a schematic diagram illustrating a system for the scalablenanomanufacturing of nanoparticles through roll-to-roll manufacturingaccording to this disclosure. The roll-to-roll manufacturing systemincludes rollers and a heating zone. An RGO paper or graphite filmembedded with micro-sized particles or carbon nanofibers coated withprecursors may be fabricated. In the heating zone, a conductive RGOpaper or graphite film is directly Joule heated or thermally radiativeheated to a high temperature, e.g., 2000-3000 K. The film may experiencefast heating and cool down after passing the heating zone. Differenttypes of nanoparticle materials may be used including metalnanoparticles (e.g., Sn and Pd), inorganic nanoparticles (e.g., Si andGe), and metal alloys. Other types of nanoparticle materials include Li,Ti, Ni, Pd, Cu, Al, Si, Sn, and other similar elements.

FIG. 5B is a schematic diagram illustrating for the scalablenanomanufacturing of nanoparticles through radiative heating androll-to-roll manufacturing according to this disclosure. The system mayinclude a rotatable member (not shown) or other structure for rotating aroll, on which a roll 512 of a carbon fiber sheet 514 is placed. Thecarbon fiber sheet 514 is passed through a precursor coating system orapparatus 516, which coats the carbon fiber sheet 514 with a saltprecursor solution. The carbon fiber sheet 514 coated with the saltprecursor solution is then passed through a zigzag drying machine 518 todry the salt precursor onto the carbon fiber sheet 514. A rapid, hightemperature thermal shock produced by radiation heating 520 is thenapplied to the carbon fiber sheet 514 having the salt precursor tosynthesize a nanoparticle on the carbon fibers of the carbon fiber sheet514. The carbon fiber sheet 514 with the nanoparticles is then rolled upinto roll 522.

FIG. 5C is a schematic diagram illustrating another embodiment of thescalable nanomanufacturing of nanoparticles through direct Joule heatingand roll-to-roll manufacturing according to this disclosure. The systemis similar to the configuration and process of FIG. 5B but uses a directJoule heating process in a chamber 550 for consecutive portions of thesubstrate sheet.

FIGS. 6A and 6B show examplary oxygen evolution reaction (OER) activityof the CoS-RGO electrocatalyst in 1M KOH. As shown in FIG. 6A, thelinear sweep of CoS 602 and RGO 604 shows that small potentials of, forexample, approximately 1.58 V and 1.66 V versus reversible hydrogenelectrode (RHE) are required to drive a 10 and 100 mA cm⁻² cathodiccurrent, respectively, which is comparable to Ir—C (20 wt % Ir),indicating the superior electrocatalytic property for OER. In contrast,pure RGO exhibits negligible current in the measured potential range,suggesting the negligible contribution of pure RGO for OER. As shown inFIG. 6B, the small Tafel slope of approximately 71 mV dec⁻¹ confirms theexcellent catalyst performance of CoS-RGO for OER.

In embodiments, CoS-RGO may be employed as an efficient bifunctionalcatalyst for both OER and HER simultaneously. As shown in FIG. 6C,CoS-RGO may be fabricated as both an anode 612 and a cathode 614 in atwo-electrode cell setup for water splitting in 1M KOH solution 615. Theelectrolysis using CoS-RGO as a bifunctional catalyst demonstratesimpressive performance for overall water splitting. Small potentials ofapproximately 1.75 V and 1.97 V are required for CoS-RGO to reach 10 mAcm⁻² and 100 mA cm⁻² cathodic current, respectively.

FIG. 6E shows an example of the long-term stability of electrolysis at afixed current density of 1 mA cm⁻² and 10 mA cm⁻² for 50 h. The voltagestabilizes at, for example, approximately 1.62 V to achieve 1 mA cm⁻²current for 10 h continuous operation, then stabilizes at approximately1.77 V to achieve 10 mA cm⁻² current for 30 h continuous operation.After switching the current density back to 1 mA cm⁻², the voltagesagain stabilize at approximately 1.62 V, indicating no structural changeof the active catalysts even after 30 h of operation under high currentdensity. This stability at different current densities without any decayillustrates the advantage of carbon-coated CoS nanoparticles on RGO foroverall water splitting.

The electrocatalytic performance of CoS-RGO is attributed to thechemical composition and electronic structure of the synthesized CoSnanoparticles grown on N and S doped RGO nanosheets. Additionally, the Nand S doping introduces defects on RGO nanosheets to form additionalcatalytic sites, and simultaneously induces the electronic interactionswith nearby CoS nanoparticles, which are beneficial to enhance thecatalytic performance for water splitting. The thin carbon coating onCoS active surface plays an important role in the corrosion resistance,resulting in impressive electrocatalytic stability.

In embodiments, the nanoparticles synthesized according to methods ofthis disclosure may be used in energy storage devices. FIGS. 7A and 7Bare graphs illustrating the electrochemical performance of athermally-shocked, synthesized RGO-Si nanoparticle (NP) composite filmas a high-capacity anode of a Li-ion battery. The RGO-Si nanoparticlecomposite films were placed in coin cells and acted as a freestandingcarbon/binder-free electrode. The electrochemical performance of theRGO-Si nanoparticle composite film was elucidated by galvanostaticdischarge-charge measurements.

FIG. 7A shows example discharge-charge curves of the RGO-Si nanoparticleanode for the first two cycles. The cell was cycled with a potentialwindow of 0.01 to 2.0 V (vs Li/Li+) at a current density of 50 mA g⁻¹.In the initial discharge cycle, the potential maintains a long flatplateau at around 0.1 V and then gradually decreases to 0.01 V. Thiscorresponds to Li insertion into crystalline Si, causing the formationof an amorphous Li_(x)Si phase. The first charge curve displays aplateau at 0.42 V, which corresponds to the delithiation process.Subsequent discharge and charge curves show characteristic voltageprofiles of Si. The overall discharge and charge capacities for thefirst cycle are 3367 mA h g⁻¹ and 1957 mA h g⁻¹, respectively,corresponding to an initial Coulombic efficiency of 58%. The initialCoulombic efficiency can be improved by compositing controlled amount ofmolten Li into the RGO/nanoparticles composites to compensate for the Liloss in the first cycle. The RGO-Si nanoparticles can deliver avolumetric capacity of 3543 mA h cm⁻³ with an areal capacity of 2.48 mAh cm⁻² at 0.13 mA cm⁻².

An example of the capacity retention of the RGO-Si nanoparticleelectrode at 200 mA g⁻¹ between 0.01 and 2.0 V is shown in FIG. 7B. Thecapacity decreased to 1165 mA h g⁻¹ over 100 cycles, which is ascribedto a small capacity decay of 0.40% per cycle. Conversely, the RGO-Sinanoparticle film delivered a small charge capacity of 126 mA h g⁻¹ over100 cycles with a severe capacity loss of 0.95% per cycle. The excellentcycling performance of the RGO-Si nanoparticle film is due to theultrafine Si nanoparticles and the RGO matrix reducing the strain andaccommodating the Si volume changes associated with thelithiation/delithiation electrochemical process.

The performance of nanoparticles in the RGO matrix can be optimized byvarying the process conditions. For example, the RGO functional groupscan be altered, and the specific hold time and synthesis temperature cancontrol the NP size.

The rapid, in situ synthesis of nanoparticles via high-temperatureradiative heating can be scaled up by three-dimensional (3D) printing,e.g., by 3D printing RGO ribbons, as shown in the example image 711 ofFIG. 7C, and RGO-SiMPs ribbons, as shown in the example image 712 ofFIG. 7C. The 3D printed ribbons may be peeled off from a substrate, suchas a glass substrate, to form freestanding and highly flexibleelectrodes as shown in images 713 and 714 of FIG. 7C. The 3D printed GOribbon is flexible as it can be bent approximately 180° as shown in theexample image 715 of FIG. 7C and the GO-Si ribbon can be wound into aspiral shape as shown in image 716 of FIG. 7C. To achieve scalablemanufacturing of high-capacity nanoparticle anodes, a potentialroll-to-roll setup can be used where freestanding RGO-nanoparticlecomposites with sufficient mechanical strength enter into thehigh-temperature thermal-shock zone for in situ synthesis ofRGO-nanoparticles. The thermal shock time (t=L/V) is determined by thelength of the high temperature zone (L) and the roll-to-roll speed (V).In embodiments, 5-15 nm Sn nanoparticles may be synthesized in a shorttime (e.g., ˜1 s), which demonstrates the potential of this method forcommercial production.

FIG. 10 is a schematic diagram illustrating particle size control bymodifying a surface of the substrates to tune the wetting behaviorbetween the substrates and the nanoparticles. For example, coating alayer of oxide on carbon substrates can make the metallic nanoparticlessmaller as compared with bare carbon substrates. In an example process,a CNF-H₂PtCl₆ or CNF-Al₂O₃—H₂PtCl₆ (CNF coated with Al₂O₃) system isJoule heated to about 1500 K for about 50 ms in an argon environment andcooled down immediately or quickly thereafter. The resultinghigh-density Pt nanoparticles dispersed uniformly on the surface of thebare CNFs were 23 nm±6.3 nm in average diameter. Employing the sameH₂PtCl₆ precursor and rapid thermal pulse parameters, the averagenanoparticle size and distribution changes with the Al₂O₃ coating (e.g.,a 10 nm Al₂O₃ coating), leading to one order of magnitude smallernanoparticle sizes and narrower size distributions (e.g., 2 nm±0.7 nm).Further details regarding particle size control are found in theappendix to the specification.

The interaction between the nanoparticles and the substrates plays arole in determining the particle morphology, distribution, andproperties. By tuning the wetting or interaction between the substratesand the nanoparticles, the design of nanoparticles with enhanceddispersion and controlled particle size may be achieved.

In embodiments, the surface modification can be achieved by coating ofan additional layer on the substrates by gas phase, solution phase orsolid phase reactions and processes. For example, atomic layerdeposition (ALD) may be used to deposit oxide layers on the substrates.The surface modifications may also be achieved by surface treatmentusing gas phase, solution phase, or solid phase reactions or processes.For example, thermal annealing in a CO₂ atmosphere may be performed tocreate surface defects.

Nanoparticles, and systems and methods for synthesizing nanoparticlesfrom micro-sized particles or salt precursors on substrates inaccordance with this disclosure are detailed above, as is theverification of these materials and methods through experimentation.Persons skilled in the art will understand that the featuresspecifically described hereinabove and shown in the accompanying figuresare non-limiting exemplary embodiments, and that the description,disclosure, and figures should be construed merely as exemplary ofparticular embodiments. It is to be understood, therefore, that thepresent disclosure is not limited to the precise embodiments described,and that various other changes and modifications may be effected by oneskilled in the art without departing from the scope or spirit of thedisclosure.

1-2. (canceled)
 3. A method of forming nanoparticles on a substrate, themethod comprising: depositing micro-sized particles or salt precursorson a substrate; and applying a rapid, high temperature thermal shock tothe substrate and the micro-sized particles or the salt precursors tocause the micro-sized particles or the salt precursors to becomenanoparticles on the substrate, wherein the micro-sized particlescomprise aluminum, tin, gold, palladium, iron, nickel, silicon, or anycombination thereof. 4-20. (canceled)
 21. A system for synthesizingnanoparticles on a substrate, the system comprising: a rotatable memberconfigured to receive a roll of a substrate sheet having depositedthereon micro-sized particles or salt precursors; a motor configured torotate the rotatable member so as to unroll consecutive portions of thesubstrate sheet from the roll; and a thermal energy source configured toapply a short, high temperature thermal shock to consecutive portions ofthe substrate sheet that are unrolled from the roll by rotating therotatable member to cause the micro-sized particles or the saltprecursors to become nanoparticles on consecutive portions of thesubstrate sheet.
 22. The system of claim 21, wherein the thermal energysource is a thermal radiation source or a direct Joule heating source.23. The system of claim 21, further comprising a rotatable memberconfigured to receive a roll for receiving a nanocomposite sheetcomprising nanoparticles on consecutive portions of the substrate sheet.24. A composite comprising: a substrate; and a plurality ofnanoparticles formed on the substrate from a micro-sized particle orsalt precursors exposed to a rapid, high temperature thermal shock. 25.The composite of claim 24, wherein the substrate is a reduced grapheneoxide substrate for the micro-sized particles or a carbon nanofibersubstrate for the salt precursors.
 26. The composite of claim 24,wherein the micro-sized particles comprise aluminum, tin, gold,palladium, iron, nickel, silicon, or any combination thereof.
 27. Thecomposite of claim 24, wherein the salt precursors comprise metalchloride, metal nitrate, metal acetate, or any combination thereof. 28.The composite of claim 24, wherein the nanoparticles are metallic,ceramic, inorganic, semiconductor, compounds, or any combinationthereof.